Pohribna Y. Formation mechanisms and microstructural characteristics of the nanocrystalline titanium created by cryodeformation

The thesis is devoted to the establishment of the physical mechanisms of the formation of the bulk nanocrystalline state of titanium VT1-0 due to cryomechanical fragmentation of grain using rolling deformation at the temperature of liquid nitrogen. A comprehensive study of the evolution of the microstructure during cryodeformation to large values (|e| = 3) was carried out by the methods of optical and transmission electron microscopy as well as X-ray diffractometry. The stepwise nature of the dependence of the grain size on the degree of cryodeformation was revealed and its correspondence to the stages of the development of mechanical twinning was shown. The limiting nanoscale average grain size (~ 35 nm), achieved as a result of cryofragmentation, and has been established. Its existence is considered as a consequence of the dependence of the twinning probability on the grain size and is explained by the absence of the possibility of twinning (the process controlling refinement) in nanosized grains. A systematic study of the characteristics of the deformation microstructure, which is formed in commercial purity titanium as a result of rolling at the temperature of liquid nitrogen at various degrees of reduction, was carried out using X-ray diffraction analysis. The relative integral intensities were determined, the size of the coherent scattering regions/crystallites and the magnitude of microstrain were estimated. The influence of the relative activity of sliding and twinning on the microstructural characteristics is discussed. The morphological anisotropy of crystallites in the nanocrystalline state formed during cryodeformation is found. Its decisive influence on the observed anisotropy of the yield stress of NC titanium has been established. The efficiency of the cryomechanical grain fragmentation process in the formation of a nanocrystalline state with an average grain size of several tens of nanometers in hcp metals (titanium, zirconium, and others) is shown. The thermal stability of the formed structural state has been established.